The decision follows secret informal discussions last Monday among about 20 institutional shareholders. The group is believed to control 20 per cent of Amstrad's equity - enough to block the proposals - but it has yet to state its views on Mr Sugar's 30p a share offer.
However, it is understood to be deeply unhappy with his terms and is poised to block the proposal at the shareholders' meeting next Thursday.
The group comprises representatives from institutions with relatively large stakes in Amstrad. At least one, which speaks for more than 10 million shares, is thought to have made a firm decision to vote against the plan. If the others follow, Mr Sugar's plans are likely to run into serious difficulty.
Separately, the Prudential and Postel, which speak for about 3 per cent of Amstrad between them, had previously announced their opposition to the buyout.
Many private investors in the company are also angry about the proposals. They have formed a shareholders' pressure group and obtained support from investors who speak for at least 2 million shares.
However, their campaign received a setback in the High Court yesterday when Justice Vinelott threw out an attempt by Edward Northcote, a shareholder, to block the buyout plan.
In his judgment, Justice Vinelott said that Mr Northcote had 'not advanced any sound reason or objection' to block an Amstrad meeting next week.
Although the judge awarded costs to Amstrad, the company said it would not be asking Mr Northcote to foot the bill.